Easiest way to block a port

Do get a firewall, but be aware that just having one isn't enough. Most firewalls are configured by default to allow no inbound traffic but all outbound traffic. So if you're trying to block an activation or serial check, or stop spyware from phoning home, you need to take the extra step to block OUTBOUND traffic across the desired port as well.

Different firewalls have different ways of handling this, but it's usually under a "Firewall" or "Rules" panel. Check the manual for the specifics on whatever model you acquire.

My personal recommendation is a Netgear ProSafe. It'll run you some extra coin, but it's a damn nice firewall with some powerful advanced features that consumer D-Link/Linksys/Microsoft routers don't usually have.

Jam it back in, in the dark.